Lysosomes- Definition, Structure, Enzymes, Types, Functions

What are Lysosomes?

  • The name Lysosome is derived from the Greek word where “lyso” means digestive and “soma” means body.
  • Lysosomes are membrane-bound vesicle which is tiny in the structure.
  • Lysosomes are involved in intracellular digestion.
  • Different kinds of hydrolytic enzymes are present in the lysosome. Under acidic conditions, these enzymes are found to be active.
  • The lumen of the lysosome is acidic which is around pH 5.
  • This condition is maintained in the membrane by the ATP-driven proton pump.
  • Low pH helps in the function of luminal hydrolases.
  • Then it degrades macromolecules and produces an amino acid, monosaccharide, and free fatty acids.
  • Lysosome helps in the digestion of the various biological materials.
  • It is also involved in the aging and death of animal cells may occur too.
  • In the liver cells of the rat, dense bodies were observed which were round.
  • The study was made through the electron microscope in the early days.
  • It was said as the perinuclear dense bodies.
  • Later in 1955, C.de duve renamed it the lysosome. It indicated a digestive enzyme that was present internally.
  • Lysosome was described as the lytic body which can lyse the cell or the tissue by the digestive enzymes.
  • In 1974, for the discovery and the research on the lysosome, Nobel Prize for physiology was awarded to de Duve. It was shared with the cell biologists, Palade and Claude.
Lysosomes
Lysosomes. Created with BioRender.com

Presence of lysosome in the Animal tissue

  • Liver
  • Kidney
  • Nerve cells
  • Brain
  • Intestinal epithelium
  • Lung epithelium
  • Macrophages(of the spleen, bone marrow, liver, and connective tissue)
  • Thyroid gland
  • Adrenal gland
  • Bone
  • Urinary bladder
  • Prostate
  • Uterus
  • Ovaries

Presence of lysosome in the Protozoa

  • Leucocytes
  • Amoeba
  • Tetrahymena
  • Paramecium
  • Euglena

Presence of lysosome in Plants

  • Onion seeds
  • Barley seeds
  • Corn seedlings
  • Yeast
  • Neurospora

Presence of lysosome in Tissue culture cells

  • HeLa cells
  • Fibroblasts
  • Chick cells
  • Lymphocytes

Structure of Lysosome

  • round
  • vacuolar
  • filled with dense material
  • single unit membrane binds it
  • variation in shape and density
  • size: 0.2 to 0.5 μm
  • size and shape differs from one cell to another and from time to time

Criteria for the identification of the cell as the lysosome

  • a limiting membrane should bind it
  • presence of two or more acid hydrolases
  • show enzyme latency

Lysosomal Enzymes

Proteases and peptidases

  • Cathepsin A,B,C,D and E
  • Collagenase
  • Peptidases

 Nucleases

  • Acid ribonuclease
  • Acid deoxyribonuclease

 Phosphatases

  • Acid phosphatase
  • Phosphodiesterase

Enzymes acting on oligosaccharide chains of glycoproteins and glycolipids

  • b-galactosidase
  • Acetylhexosaminidase
  • b-Glucosidase
  • Glucosidase
  • a-Mannosidase
  • Sialidase

Enzymes acting on glycosaminoglycans

  • Lysozyme
  • Hyaluronidase
  •  b-Glucuronidase

 Enzymes acting on lipids

  • Phospholipase
  • Esterase

 Lysosomal Membrane

  • As compared to the mitochondria, it is slightly thicker.
  • Sialic acid is present in it.
  • Since the lysosomal membrane protein is highly glycosylated, it protects from the action of the lysosomal proteases.
  • The lysosomal membrane can fuse with the other membranes of the cell which is the unique property.
  • When the lysosomal membrane ruptures, lysosomal enzymes are released.
  • It can be caused by the destabilizing influence of the surface-active agents and the steroid sex hormones.
  • The lysosomal membrane is stabilized by cortisone and hydrocortisone.
  • On the tissue, they possess an anti-inflammatory effect. 
  • Within the lysosome all the process of digestion takes place.
  • For the action of the lysosomal enzyme, the medium should be acidic.
  • To maintain the acidic condition inside the organelle, there should be an accumulation of the H+.
  • It is maintained by the proton pump which is ATP-dependent.
  • Transport protein is also present in the lysosomal membrane.
  • When the macromolecules get digested, the final products can be transported by these proteins.
  • After the transportation, they can be further utilized by the cell or be excreted.

Types of Lysosomes (Polymorphism in lysosomes)

  • Polymorphism can be seen in the morphology of the lysosome.
  • There are four types of lysosomes. They are:
    • Primary lysosome
    • Heterophagosomes
    • Autophagososomes
    • Residual Bodies

A. Primary Lysosomes

  • They are also called:
    • Storage granules
    • Protolysosomes
    • Virgin lysosomes
  • The primary lysosome is bounded by a single membrane.
  • It has a diameter of 100nm.
  • A digestive enzyme is present in it which has not taken part in the digestion.
  • One type of enzyme or another is present in it.
  • Only in the secondary lysosome, there is the presence of the full complement of acid hydrolases.

B. Heterophagosomes

  • They are also called:
    • heterophagic vacuoles
    • heterolysosomes 
    • Phagolysosomes
  • When the primary lysosome fuse with the cytoplasmic vacuoles, a heterophagosome is formed.
  • Extracellular substances are present in the cytoplasmic vacuoles.
  • Different endocytic processes like pinocytosis, phagocytosis, or receptor-mediated endocytosis help in bringing such extracellular substances into the cell.
  • In these secondary lysosomes, there is the presence of the hydrolytic enzymes which digest the engulfed substances.
  • After digestion, such particles will pass across the membrane of the lysosome.
  • Then it will become part of the matrix.

C. Autophagosomes

  • They are also called:
    • Autophagic vacuole
    • cytolysosomes 
    • Autolysosomes
  • Digestion of the different intracellular structures like mitochondria, ribosome, peroxisome, and glycogen granules can be done by the primary lysosome.
  • Autophagy is called autodigestion.
  • During cell growth and repair, autophagy is a normal event.
  • It is prevalent in the differentiation, the dedifferentiation of the tissues and tissue under stress.

Autophagy occurs in different forms:

a. By fusion of the lysosome:

  • It is enclosed inside the double membrane sac.
  • Then there is the breakdown of the inner membrane.
  • Penetration of the enzyme can occur to the enclosed organelle.

b. Formation of a vesicle and fusion with primary lysosome:

Microautophagy also takes place. When the digestion process proceeds then it becomes difficult to identify the type of secondary lysosome if it is heterophagosome or autophagosome. So, in this stage, it is said as the digestive vacuole.

D. Residual Bodies

  • They are also called:
    • Telolysosome
    • Dense bodies
  • Incomplete digestion results in the formation of the residual bodies.
  • When some lysosomal enzymes are absent, incomplete digestion may occur.
  • Inside the digestive vacuoles, the undigested food remains as the residue.
  • Then they make take different forms.
  • Residue body is larger and irregular in shape.
  • By the defecation, residual bodies are eliminated in the case of Amoeba and some other protozoa.
  • In some cells, for a longer period, the residual body may stay which may cause aging.

Functions of Lysosomes

  • Lysosomes digest the large extracellular particles. Different substances which are presented by the phagosomes or pinosomes are digested.
  • Lysosomes digest the intracellular substances. The stored food particles like the glycogen, lipids, and proteins, during starvation, are digested by the lysosome.
  • Autolysis or cellular autophagy occurs in some pathological conditions. There is the liberation of the enzymes from the lysosome after the death of the cell. Dead cells are then digested by the enzyme.
  • In the cells like the sperms, the lysosome helps in extracellular digestion. The membrane of the ovum is digested which aids in the penetration of the sperms in the ovum. 

References

  1. Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S.Chand and Company Ltd.
  2. Perera, R. M., & Zoncu, R. (2016). The Lysosome as a Regulatory Hub. Annual Review of Cell and Developmental Biology32, 223–253. https://doi.org/10.1146/annurev-cellbio-111315-125125 
  3. Jezegou A, Llinares E, Anne C, Kieffer-Jaquinod S, O’Regan S, et al. 2012. Heptahelical protein PQLC2 is a lysosomal cationic amino acid exporter underlying the action of cysteamine in cystinosis therapy. PNAS109:E3434–43
  4. Liu B, Du H, Rutkowski R, Gartner A, Wang X. 2012. LAAT-1 is the lysosomal lysine/arginine transporter that maintains amino acid homeostasis. Science 337:351–54
  5. Sagne C, Agulhon C, Ravassard P, Darmon M, Hamon M, et al. 2001. Identification and characterization of a lysosomal transporter for small neutral amino acids. PNAS 98:7206–11.

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